Nuclear transport is the import or export of proteins across the nuclear pore complex (NPC). Nuclear transport proteins are involved in the transport process and they determine the nuclear transport capacity of a given cell. The transport capacity may have a direct impact on gene expression, signal transduction and cell growth and development.
Karyopherin beta (β) 1 (Kpnβ1) is a nuclear transport protein involved in the import of cargo proteins and RNAs across the NPC, from the cytoplasm into the nucleus. Kpnβ1 is a member of the Karyopherin β superfamily of nuclear transport proteins. There are over twenty members of the Karyopherin β protein family, which can function as either import or export receptors, mediating either the nuclear entry or exit of proteins. Kpnβ1, also known as Importin β, is a major nuclear import receptor in the cell that transports proteins containing a nuclear localisation signal (NLS) through the NPC into the nucleus. Kpnβ1-dependent nuclear import is typically characterised by the recognition of the NLS on the cargo protein by the Kpnβ1 adaptor protein, Karyopherin α (Kpnα), also known as Importin α. After cargo recognition, Kpnα binds Kpnβ1 and the trimeric complex translocates into the nucleus.
The Crm1 inhibitor Leptomycin B (LMB) is the only accepted and commercially available compound known to inhibit nuclear transport. Recently, small molecule peptidomimetic inhibitors of Kpnα/β-mediated transport were identified in an in vitro screen, however, these inhibitors have low potency and are not cell permeable, and hence no inhibition of Kpnα/β-mediated nuclear import could be observed in vivo (Ambrus G, Whitby L R, Singer E L, Trott O, Choi E, Olson A J, et al. Small molecule peptidomimetic inhibitors of importin alpha/beta mediated nuclear transport. Bioorg Med Chem 2010 Nov. 1; 18(21):7611-20).
Peptide inhibitors that bind Kpnα with a strong affinity have also been described, yet these do not inhibit Kpnβ1 directly (Kosugi S, Hasebe M, Entani T, Takayama S, Tomita M, Yanagawa H. Design of peptide inhibitors for the importin alpha/beta nuclear import pathway by activity-based profiling. Chem Biol 2008 Sep. 22; 15(9):940-9). Ivermectin is a broad-spectrum anti-parasitic that inhibits the Kpnα/1 complex, but it does not appear to block import mediated by Kpnβ1 alone (Wagstaff K M, Sivakumaran H, Heaton S M, Harrich D, Jans D A. Ivermectin is a specific inhibitor of importin alpha/beta-mediated nuclear import able to inhibit replication of HIV-1 and dengue virus. (Biochem J 2012 May 1; 443(3):851-6). Karyostatin 1A (Hintersteiner M, Ambrus G, Bednenko J, Schmied M, Knox A J, Meisner N C, et al. Identification of a small molecule inhibitor of importin beta mediated nuclear import by confocal on-bead screening of tagged one-bead one-compound libraries. ACS Chem Biol 2010 Oct. 15; 5(10):967-79) and Importazole (Soderholm J F, Bird S L, Kalab P, Sampathkumar Y, Hasegawa K, Uehara-Bingen M, et al. Importazole, a small molecule inhibitor of the transport receptor importin-beta. ACS Chem Biol 2011 Jul. 15; 6(7):700-8) are the first small molecule inhibitors of Kpnβ1 to be identified, however, their off-target effects have not yet been examined. Moreover, no inhibitor of nuclear import has as yet been tested for its anti-cancer effects. There thus remains a need for the identification of novel and effective Kpnβ1 inhibitors and for the inhibitors to be tested for their anti-cancer activities.
The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.